There are cheap amps and expensive amps, what are the differences and what are things to look for when buying an amplifier.

Power
Obviously the first thing that comes to mind is the amplifiers actual output.
This is shown in two way's RMS and MAX power.

RMS is short for Root Mean Squared and refers to the amplifiers ability to produice a certain power level on a continous basis. Typically this figure is derived by getting the amplifier to play pink noise (every frequency at once) into a load continously.

MAX (Maximum) refers to the amplifiers ability to produice a certain power level for a split second burst. The problem with this measure is there is no uniform means of measurement so it really means nothing. Some amps will claim 200 RMS & 500watts max others will be 200RMS and 1000watts max

Never use the MAX power figure as a comparison between amps

Watch OUT
Even RMS figures can be manipulated so be carefull..
They can be quoted at various voltages 12v, 13.8v or 14.4v. Because most car audio amplifiers have unregulated power supplys they will produce more power with higher voltages so cheap amps will often quite figures at 14.4v. (Alpine do this and it really anoyes me)
13.8v is a realisitic volotage for a running car so compare power at 13.8v or 12v not 14.4v
Also ensure the RMS is with all frequency's playing (i.e. 20-20,000htz) not at 1k
Look for
200rms @ 13.8v (20-20,000htz) NOT
200rms @ 14.4v (1k) (

The difference in the above example may be as much as 50-80watts rms.

Cheap amps will claim RMS figures under the best possible conditions and may strech the actual figure achieved. Quality amps will be conservative in their mesaurements and will quote figures under realisitic conditions. Some quality amps will actually produce 50% more than there claimed RMS power. So don't think just because the Rockford Fosgate amp your looking at claims 1000ms @ 1ohm & the BOSS amp also claims 1000rms @ 1ohm they will produce the same power figure.

Dampening (or Dampening Factor) = The ability of the amplifier to control the movement of a speaker. The higher the number the better and anything over 400 is good.

Signal to Noise STN = is the ratio of signal theat the amp amplifies to the amount of noise / interference it introduces. Signal to noise is most noticable on amplifiers running front speakers and less noticable on amps running subs.
The higher the STN the better.
Over 110db is awesome
Over 100db is very good
Over 90db is ok
As a rule i would try to ensure all amps are over 90db

What are ohm's and do I need to look at this when buying an amp?

An Ohm is a measure of resistance. The greater the resistance of a speakers voice coil the more it resists power being fed through it. Most home audio speakers and subs are 8ohm's while car audio is mostly 4ohm's. Resistance is a critical part of two pieces of equipment
- Amplifiers
- Speakers / Subs

With speakers the greater the resistance the more it resists power being fed into it
Amplifiers produce certain amounts of power for a given resistance. The greater the resistance from the speaker the less power the amp will provide to it. A prefect amplifier will doubble it's power when resistance is halved but in the real world many other factors prevent this from happening such as the size of the power supply, voltage supply, and heat.

If amplifiers produce more power at lower resistace why aren't all speakers low resistance like 2ohms?
As resistance decreases so does the dampening factor which, as discussed, is the amplifiers ability to control the movement of a speaker. So while the amplifier produces more power with the lower resistance it also looses some of its control over the cone movement causing subs to sound less tight and punchy. Amplifiers also get much hotter at lower resistances so heat dissopation become important so as not to fry the amps internals. Good ventelation and cooling fans can assist here. They also need larger internal power supplys, capacitors, mosfets etc

Different Classes of Amps
There are a number of different types of amps and I will not go into detail on all for all them but by far the 2 most common are Class A/B and Class D.

Class A/B are typically 2, 4 or 5 channel amps that can run speakers or subs and are the most common amplifier type available. They have an efficiency of around 50% so for the power they draw 50% of it gets converted to power your speakers/subs while the other 50% is lost as heat

Class D is a relitavely new technology and these amps are commonloy refered to as monoblocks as they ae often only 1 channel. The advantge they have over class A/B amps is that they are much more efficient typically around 80-85% meaning they are much smaller and produce a lot less heat for the same power output. They also draw less current and work into work into much lower impedences.

The drawback is they produce noise/hiss and have lower signal to noise ratios but this is normally not noticed as they are primarily used to drive subwoffers and the noise is filtered out by usng crossovers to restrict the amp to playing frequencies under 200htz. A common system configuration is to have one or more Class A/B amps running front and rear speakers with one or more monoblocks driving the sub(s).

Other things to consider - Simple things can make a big difference
- Amplifers disserpate heat through heat sinks or fans,. It is important that the heatsinks are well ventelated. If you MUST mount an amplifier in a restricted space such as behind a pannel or close to a parcel shelf then consider mounting a fan nearby to ensure constant air flow over it. Even a small amount of airflow makes a BIG difference

- Amplifiers need power/earth connections & signal input & speaker outputs. Consider where you will be mounting the amp in your system and buy noe that has terminals located for easiest and neatest connection. Some amps have all connections on one end others have them at both ends

- Consider the cable mounting methods,. Better quality amps will have gold plated connectors and may offer better cable mountnig methods such as allen key screw down terminals that accept larger cable sizes. They may also include mounting accessories such as end caps to cover wiring and neaten the install.

- Features: many amplifiers offer features such as on board crossovers, bass boost, remote gain controls, remote voltage monitoring devices, etc. All of these things allow for greater flexability in install and down the track plus provide for higher re-sale value later on.

- Markings and Controls: Look for amps that offer controls (gain / crossovers etc..) that are easy to get to, easy to operate and well marked. A crossover with markings every 50htz is much better than one that has a 50htz mark then and the next mark at 400htz. How are you supposed to accurately set it at 120htz?

- Cosmetics: Is a personal preference area but if your going to make a feature of the amp and/or sub in the boot then ensure they have something visual to offer. Many have nice chrome faces or backlit logos etc

- Protection features: Better amps offer better protection such as low voltage, overheating, low impedence etc. These will shut the amp down until you have rectified the problem (ie the amps cools down or you change the speaker load). One of the biggest problems with cheap amps if they offer little or poor quality protection circuts so when you do something wrong they are not very forgiving. A quality amp knows your been a doofus and tried to hook up you and your mates 6 subs in series to it so it protects itself from destruction.